In flexbeam helicopter rotors, it is important that the flexbeam be light in weight, be capable of withstanding substantial loads such as the centrifugal loads imposed thereon by the helicopter blade during helicopter operation, and reacting various motions such as the helicopter blade pitch change, lead-lag, flapping and torsional motions. A flexbeam made of composite materials, in particular, a plurality of high tensile strength lightweight fibers bonded together with a resin while extending in unidirectional fashion for the full length of the ply is particularly attractive in this environment. Two dog-leg shaped plies of such composite material can be formed and positioned mirror image to extend either for the full length or a portion of the length of the flexbeam leg, in combination with other composite plies, and then bifurcate in passing through the flexbeam hub and extend either for the full length or a portion of the length and with each of the dog-leg plies occupying half of the width of two substantially oppositely positioned legs. This results in very favorable load transfer, particularly centrifugal load transfer, between the various legs of the flexbeam, and also permits the accommodation of the required motions because of the flexibility of the flexbeam members. Such a flexbeam and its method of manufacture are described in U.S. application Ser. No. 07/641,237 filed on even date herewith, in the name of Doolin et al and entitled "A Unitary, Multi-Legged Helicopter Rotor Flexbeam Made Solely of Composite Material and the Method of Manufacturing Same" and U.S. application Ser. No. 07/641,220 now U.S. Pat. No. 5091029 filed on even date herewith, in the name of G. Davis et al and entitled "Method of Manufacturing a Unitary, Multi-Legged Helicopter Rotor Flexbeam Made Solely of Composite Materials", all assigned to a common assignee.
Accordingly, it is very important to be able to produce such dog-leg plies inexpensively, rapidly, and in production quantities in the fabrication of such flexbeam helicopter rotors.
It is known in the prior art to produce such plies of fibers by winding them onto a rotating mandrel. For example, in German Patent No. 3,243,519 a cylindrical mandrel is shown, and rhomboid shaped projections are positioned axially therealong. As understood, the teaching of the German patent is to wind fibers or filament onto the mandrel in the areas between the rhomboid projections so as to form curved spring rods. It is important to note, however, that the German patent does not wind filament over the rhomboid shaped projections but rather winds the filament between the projections into the voids formed there-between. U.S. Pat. No. 3,472,718 to W. P. Siegmund, granted Oct. 14, 1969, on "Methods of Making Display Devices" fabricates a display devices which consists of several stacked shelves, some of which are rectangular in shape and which are formed by winding filament onto a cylindrical mandrel, and some of which are arcuate in shape and which are formed by winding filament onto a conical mandrel. Siegmund therefore is capable of producing both straight filaments and arcuate filaments, but not the combination of the two. To produce a dog-leg shaped filament following the Siegmund teaching, it would be necessary to join straight filament plies to the opposite ends of an arcuate filament ply. This would not provide a dog-leg ply having high tensile strength fibers extending for the full length thereof, but would rather include joints of resin or other bonding material between the opposite ends of the arcuate ply and the two straight plies. These resin joints would be unable to withstand high tensile forces such as the centrifugal force of a helicopter rotor in operation. Contrary to this, we teach producing a dog-leg ply having high strength filaments extending unidirectionally for the full length thereof so that the ply has high tensile strength qualities and is able to transmit high tension loads therethrough, for example, in connection with the operation of the flexbeam helicopter rotor.